They don't work. I was part of a group on netmums that went off to do our own group and my God it got nasty really quickly.
I am in a lovely group on Facebook of people from netmums which has been running for a few years and its wonderful. We all get on well and its very supportive. That's where we put stuff we don't want made public. It works well.
I couldn't cope with another forum. I have enough trouble keeping up on here, Facebook and SimCity.